Hollow aerofoil blades are used in gas turbine engines. A typical method for manufacturing hollow aerofoil blades involves the use of diffusion bonding and superplastic forming techniques as described generally in U.S. Pat. No. 5,469,618, for example.
In a typical method, an assembly 1 is provided comprising a pair of mutually opposing panels 2 as shown in cross-section in FIG. 1A. The dashed line in FIG. 1A represents the interface between the respective panels 2. The panels 2 are typically diffusion bonded together to form an integral body.
However, a region of one or both of the faces of the panels 2, at the interface 3, is typically printed with a stop-off material such as yttria, or yttrium oxide, to prevent bonding in said region. For example, whilst the edges of assembly 1 may bond to form an integral body, a region in the centre of the assembly 1 does not bond.
The bonded assembly is arranged in a cavity die 4, and a superplastic forming process is performed to expand or inflate the assembly 1 to conform to the cavity die 4 and to form an internal hollow cavity within the assembly 1 in the unbonded region, as defined by the printed yttria.
This technique is generally known in the art.
For certain applications, it is desirable for the internal surface of the hollow aerofoil blade to include a series of ribs projecting into the internal cavity and extending generally in the direction from the leading to trailing edges of the blade. These ribs are typically incorporated in order to provide some structural rigidity to the blade and also to provide constricting elements for preventing the movement of visco-elastic damping material within the internal cavity. The visco-elastic damping material is typically injected into the internal cavity after the superplastic forming process is ended, and is present to damp vibrations within the aerofoil blade during use.
In order to form the ribs in the internal cavity of the final blade, the panels 2 are initially provided with correspondingly shaped ribs 5 on their outer surfaces, rather than on the inner surfaces which form the interface between the panels in the assembly 1. During the superplastic forming process, as the assembly is inflated, the outer surfaces of the panels 2 of assembly 1 conform to the internal structure of the cavity die. In so doing, the ribs are effectively transferred, or transposed, from the outer surfaces of the panels to the inner surfaces of the panels, and thus project into the internal cavity 6 formed between the panels during the superplastic forming process.
The resulting structure of the blade, after the superplastic forming process, is shown in FIG. 1B.
As can be seen, the internal cavity 6 is formed between the respective panels 2 which are diffusion bonded at least at their periphery, e.g. as in region 7. And ribs 5 have been transposed from the outer surfaces of the panels 2 (as shown in FIG. 1A) to the inner surfaces of panels to project into the internal cavity 6 (as shown in FIG. 1B).
Accordingly, when the visco-elastic material is located in the internal cavity 6, the ribs 5 act to restrain the material and help to prevent its movement within the internal cavity during use of the blade.
As will be appreciated, when the blade is provided on a rotor of a turbomachine, e.g. a gas turbine engine, the blade is rotated at very high angular velocities. Accordingly, the visco-elastic materials located in the internal cavity 6 are urged to creep along the internal cavity blade towards the tip of the blade. The constriction provided by mutually opposing pairs of ribs is provided to help prevent this creep.
However, it is important to regulate the size of the gap 8, or throat, between mutually opposing pairs of ribs (i.e. opposing ribs provided respectively on the panels 2). The gap 8 can be seen in FIGS. 2A and B for example.
This is because, on the one hand, if the gap 8 is too large, then the visco-elastic material is able to effectively migrate through the gap and move around within the internal cavity 6 as the blade is moved (during use), which is undesirable. The upper limit x1, as shown in FIG. 2A, of the size of the gap 8 largely can be controlled by suitable definition of the relevant dimension y of the cavity die 4.
Whereas, on the other hand, if the gap 8 is too small then it can effectively pinch off the visco-elastic material and cause fractures therein, especially when the visco-elastic material is under tension during rotation of the blade. It is difficult to control the lower limit x2 of the gap 8 by suitable definition of the dimensions of the cavity die 4. Under venting and cooling conditions it can be difficult to reliably reproduce gaps 8 having suitable lower limits x2.
In addition to the problem of pinching described above, in some circumstances, mutually opposing ribs 5 may define only a minimal gap 8, or even no gap at all. In some circumstances the ribs 5 of a mutually opposing pair of ribs may even abut against each other as shown in FIG. 2B. This is problematic because such an arrangement of the ribs will then prevent injection of the visco-elastic damping material into the entirety of the internal cavity 6 of the blade during subsequent processing, resulting in a sub-standard blade in which an insufficient amount of damping material is provided within the internal cavity.